If you have a playlist of videos, video sync prefetches the information it needs using YouTubeDL for every single video at the exact same time. This causes VRC to lock up while that is happening. And the severity of the lockup is proportionate to how many videos are in the playlist.

This destroys the ability to have a long mix of videos in your playlist.

@Aev: It's still happening, actually. It no longer lags you out when you join the world, but it still takes many minutes to start playing the first video. Turns out it's still doing a HTTP HEAD request per video before it will even let you play the first one.

On top of that, at least 90% of the time, it doesn't even start playing the first video after it finishes all that.